1. Field of the Invention
The present invention generally relates to liquid crystal display panels and, more particularly, to backlighting of nematic curvilinear aligned phase (NCAP) liquid crystal display panels to improve contrast of the display which is particularly suited for automotive applications.
2. Description of the Prior Art
Liquid crystal displays (LCDs) are commonly used for a variety of applications ranging from personal televisions and computer displays to automotive applications. One of the principal reasons for the popularity of LCDs is their small size and low power consumption. On the other hand, the contrast of the display is limited and the angle at which the display must be viewed is critical for many LCDs which is a hindrance especially for automotive applications. Many LCD panels are dependent on front lighting in order to achieve significant contrast between the portion of the display that is ON and the portion that is OFF. The LCD material provides about 70% transmission in the ON state and 30% transmission when in the OFF state. This ratio (about 2.3) is not adequate for most purposes. A way the contrast is made useful is to cause the light to travel twice through the LCD by illuminating it from the front and reflecting the light back through the LCD to be viewed from the front. In this way 0.7 squared, or 0.49, is obtained when the panel is ON and 0.3 squared, or 0.09, is obtained when OFF for a ratio of 5.4, which is adequate. If slightly lower ON transmission is satisfactory, the ratio can be increased markedly. Using a translucent reflector in back of the LCD makes it possible to back light the panel but does not give sufficient contrast since the light only travels once through the LCD. Thus, backlighting of these single panel displays has been unsatisfactory.
An approach used in the prior art to address the problem of contrast in liquid crystal displays under a variety of lighting conditions is to provide a dual panel display with two modes of operation. For example, U.S. Pat. No. 4,516,834 to Cascini discloses a contrast enhanced LCD comprising two substantially identical LCDs with a transflector disposed in between such that light entering from the viewing surface of the first LCD is attenuated twice by the dark areas of the first LCD, and light entering from the rear of the second LCD is also attenuated twice, once by the rear LCD and once more by the front LCD. An electroluminescent panel is used as a backlight source when the display is used in a transmissive mode under poor ambient light conditions.
In another example, U.S. Pat. No. 4,637,687 to Haim et al discloses a cascaded, dual cell transflective LCD which comprises a pair of LCDs spaced from one another and having a transreflective element therebetween. A backlight source is positioned behind the second LCD and energized when the cascaded dual LCD is operated in its transmissive mode. In its reflective mode, ambient light passes through the front LCD and is reflected by the transreflective element to pass through the LCD to the viewer. Thus, the degree of contrast ratio in the reflective mode is determined by the degree of light absorption as the light passes through the unenergized background portions of the LCD twice. In the transmissive mode, light from the backlight source passes through the two LCDs, thereby providing the same contrast ratio as in the reflective mode.
U.S. Pat. No. 4,693,560 to Wiley discloses a double layer LCD which comprises first and second LCDs spaced from one another and a transflector disposed therebetween. The two LCDs comprise a liquid crystal material containing a dye that conforms to the structure of the liquid crystal material and a containment medium for inducing distorted alignment of the liquid crystal material which, in response to such alignment, scatters and absorbs light. A backlight source is placed on the nonviewing side of the second LCD. The Wiley display also has two modes of operation. At night, the display acts as a totally transmissive display wherein the contrast and brightness of the display are determined by the product of the performance of the two LCDs. In bright sun, the backlight source is turned OFF and the contrast ratio and brightness of the display are products of the double pass of incident light through only the front LCD.